I would think long and hard about getting a Rock Python. Rock's have the size of a Burm (if not bigger) and not half the sweet temperament. Don't get me wrong, I've heard of puppy dog sweet Rock's... but they're few and far between.

Temperament aside, the cost of feeding a full grown Rock is going to be high, as well as getting a large enough enclosure for him. You'd probably have to make one yourself.

Don't jump into something this big without doing a TON of research. If you still feel you're up to the task, then go for it... but that poor Rock has already been abandoned. Don't take him in unless you're planning for the long haul.